Is there a particular way to track healing done to an enemy target? I know that I can filter the combat log for particular heals, but I am unaware of a method of filtering for generic healing done to a target in a way that returns a healing done sort of value.
When the event fires, check if it's a message type you're interested in (SPELL_HEALING or SPELL_PERIODIC_HEALING) and related to a target you're interested in (check the GUID).
As for time, every combat log message includes a timestamp. When one comes in, check if it's been more than <amount of time> since you started watching. If it's been more time than you want to watch, stop watching.